The present invention relates to an image forming apparatus such as a full-color copying machine, a color printer, or the like.
A so-called train-of-four tandem full-color copying machine is known as an image forming apparatus which outputs a color image. In this train-of-four tandem full-color copying machine, four image forming units which respectively form toner images of colors of yellow (Y), magenta (M), cyan (C), and black (B) are provided in parallel along a conveyor belt, based on an image signal subjected to color separation. A DC motor is adopted as a drive system thereof.
As shown in FIG. 1A, the train-of-four tandem full-color copying machine as an image forming apparatus according to the prior art generally adopts a xe2x80x9c4-motor systemxe2x80x9d in which four photosensitive drums 100Y, 100M, 100C, and 100B are respectively driven by motors 101Y, 101M, 101C, and 101B.
In contrast, as shown in FIG. 1B, it is possible to adopt a xe2x80x9c1-motor systemxe2x80x9d in which four photosensitive drums 100Y, 100M, 100C, and 100B are all driven by one motor 102.
In case of adopting this 1-motor system, an electronic clutch or a mechanical 1-way clutch is adopted to switch a color mode in which the four photosensitive drums 100Y, 100M, 100C, and 100B are all driven and a black mode in which only the black photosensitive drum 100B is driven. The 1-motor system provides a merit, compared with the 4-motor system.
However, there is a problem that image jitters may occur depending on the precision of mesh at the clutch part. Therefore, gears with a highly precise mechanism are required in the case of adopting the 1-motor system.
Meanwhile, as shown in FIG. 1C, a xe2x80x9c2-motor systemxe2x80x9d can be adopted, i.e., three photosensitive drums 100Y, 100M, and 100C are driven by one motor 103 through a gear, a belt, or a roller and the remaining black photosensitive drum 100B is driven by another motor 104.
If the 2-motor system is adopted, the clutch part can be omitted, so that image jitters caused by mesh at the clutch part can be avoided.
In the 2-motor system, however, the load torque ratio is 3:1 under the condition that the drive gear ratio is constant. The load difference is thus large and causes a problem.
If two motors of the same kind are adopted and if the drive gear ratio is changed to match the load torque ratio, the target peripheral velocities differ from each other so that the frequency generator (FG) pulse frequencies generated per one rotation differ greatly. Therefore, it is difficult to achieve control with high precision for synchronizing rotations of the two motors 103 and 104 in the FG cycle. Also, a difference occurs between peripheral velocities at the rising/falling times of the motors 103 and 104 due to the gear ratio. Consequently, it is difficult to match the peripheral velocities with a contacting transfer belt.
The present invention has been made in view of the above problem and has an object of avoiding the problem of high costs when the 4-motor system is adopted and the problem of mesh at the clutch part when the 1-motor system is adopted, by adopting a 2-motor system.
That is, photosensitive drums for color components Y, M, and C are driven by one motor through a gear, belt, or roller, by adopting the 2-motor system. The photosensitive drum for the other remaining color component B is driven by another motor. In this structure, a cost-reduction and preservation of image characteristics are both achieved by reducing the clutch part.
Further, an object is to prevent a difference from occurring in the peripheral speed at the rising/falling time between the motor for driving the photosensitive drum for the color component B and the motor for driving the photosensitive drums for the color components Y, M, and C, by controlling the drive current for driving the photosensitive drum for the color component B, to delay the rising time of the motor for driving the photosensitive drum for the color component B, even if two motors respectively having different numbers of FG pulses are used.
To achieve the above objects, an image forming apparatus according to the present invention comprises: a conveyor section which conveys an image forming medium; an image forming unit having a plurality of photosensitive drums for color components of yellow, magenta, cyan, and black arranged in parallel on the conveyor part and being rotatable, to transfer images of respective colors to the image forming medium being conveyed by the conveyor part; a first motor which drives all of the photosensitive drums for the color components of yellow, magenta, and cyan through a drive mechanism; a second motor which drives the photosensitive drum for the color component of black; a reference clock generation circuit which generates a reference clock for driving the motors; a first motor control circuit which drives and controls the first motor; a second motor control circuit for driving and controlling the second motor; and a reference clock multiplication/division circuit which multiplies/divides the reference clock, to output a result to the second motor control circuit.
Further, a method of forming an image according to the present invention is based on a two-motor system in which photosensitive drums for color components of yellow, magenta, and cyan are driven by a first motor and a photosensitive drum for a color component of black is driven by a second motor, and the method comprises: a first step of conveying an image forming medium; a second step of multiplying/dividing a reference clock to output a clock to a second motor control circuit for driving and controlling the second motor; and a third step of driving the first motor by a first motor controlling circuit based on the reference clock, to drive the photosensitive drums for the color components of yellow, magenta, and cyan, and of driving the second motor by the second motor control circuit based on the multiplied/divided clock, to drive the photosensitive drum for the color component of black, thereby to transfer images of respective colors to the image forming medium.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.